But it didn’t help that he was trying to medicate ulcerative colitis in secret and could barely eat.

He transferred to the Marines. Vomiting daily, he wrote it off as the stress of being a rifle platoon commander. In November 2014, he wound up hospitalized. “I drank this radioactive fluid that would attach to all the cancerous cells in my body,” he recalls, “and I was a Christmas tree.” Cancer had exploded across every organ in his abdomen.

It was Burkitt non-Hodgkin lymphoma, the fastest-growing human tumor, and it was in an advanced stage, IV-B, which sounded to him like “one step from dead.” The demographics were puzzling. Most often Burkitt strikes children in Africa; next come people who are HIV-positive. “That scared me even more, somehow,” he admits. “Did I have AIDS, too?”

But Casey’s lymphoma was a third form, linked to his immune system. Because, as an undergraduate studying biology at Washington University, he’d volunteered in a research lab, he startled the military doctors by asking things like “Is it the p53 tumor suppressor gene that failed?” Later he’d form his own theory: All those steroids he’d taken for the colitis would have suppressed his immune system. “Add a genetic predisposition,” he says, “and it was like telling the castle guards, ‘Put down your arms.’”

With intense chemo Casey had about a 50 percent chance of recovery. F—k it, he thought. Let’s do this. Already lean, he lost 70 pounds during treatment. He remembers “waking up to the sound of my parents crying.” He started envisioning his thoughts as carousel horses, his brain spinning so fast he couldn’t jump on one and ride it.

But on May 18, 2015, his cancer was officially pronounced in remission. In August he went skydiving, freefalling into a cloud.

First he’d been driven by misery.

Then by hope.

This summer Casey interned at a National Institutes of Health cancer research lab, studying the very cancer that attacked him. At press time he was competing in the Mongol Derby, a 1,000-kilometer horse race across the Mongolian steppe.

Casey isn’t even an experienced horseback rider—he found a horse to teach him to ride. The Mongol Derby, after all, is mainly about endurance.